High velocity oxygen-fuel (HVOF) thermal spray torches are known for use in applying a metallic coating to the cylinder bores of an engine block. See, for example, U.S. Pat. Nos. 5,014,916 and 5,080,056. Part of the HVOF system of the type which the invention is concerned is the nozzle. The nozzle serves to guide a feed wire to a high temperature, high velocity combustion zone developed by a high velocity mixture of oxygen and gaseous fuel. The gases are fed through the nozzle and are combusted in the combustion zone which melts the tip of the feed wire. The molten material is subsequently atomized as it is discharged from the torch at high velocity against the walls of the cylinder bores.
Commercially-available nozzles for such HVOF wire feed systems use either a fully mixed flow of oxygen and fuel that is directed into the combustion chamber where it is burned (fully mixed), or provide separate flows of oxygen and fuel that are introduced into the combustion chamber where mixing of the gases and ignition occur simultaneously (external mixing type). While the fully mixed-type nozzle produces a desirable high temperature flame, such an arrangement is subject to flashback wherein combustion propagates from the combustion chamber up the nozzle and into the mixing chamber where it is prone to damaging seals and other hardware of the nozzle. The external mixing type of nozzle avoids the problem of flashback, but at the expense of performance. The simultaneous mixing and combustion of the individually delivered gases operates at a lower temperature and consumes the feed wire at a lower rate, decreasing the deposition rate of the material.
Another drawback to known HVOF systems is that the package size of the nozzle is fairly bulky due to the arrangement of the gas delivery tubes and wire feed tube, which must be dealt with when coating cylinder bores, and in some cases could limit its application based on size.